a general idea of the kind of packages you want to maintain (basically, why do you want to become TU?)

Even though you could become a TU by merely fulfilling those minimum requirements, the people judging you during the standard voting procedure might expect more from you. Such as:

involvement in the bug tracker (reporting, research, info)

patches for Arch projects

involvement in a few open-source projects (even if they are your own)

If you still feel up to becoming a TU after reading these lines, the first step is to find a TU who agrees to sponsor you. Once sponsored, you should write a witty application signed with your GPG key to the aur-general mailing list.

Note: Should the TU you contact decline to sponsor your application, you should make this fact known if you seek sponsorship from another TU.